1. FIELD OF THE INVENTION
This invention relates to the field of communication transmissions and in particular to the acquisition and tracking of timing phase information.
2. BACKGROUND ART
Communication of data information over the voice network between a sending and receiving station is typically accomplished with a modem (modulator/demodulator). A modem transforms digital information by modulating it according to one of several well known modulating schemes. This modulated data is transmitted to the receiving modem, where it is demodulated to produce the original information. To properly demodulate and decode transmitted information, the clock rate of the transmission must be known to the demodulator. Typically, timing information is taken from the transmitted signal itself by means of various techniques.
One prior art transmission method utilizes a "header" of set up and timing information transmitted in quadrature phase shift key (QPSK) mode followed by actual data transmitted in quadrature amplitude modulation (QAM) mode. In the prior art, timing signal acquisition is accomplished by determining maximum or minimum baud energy in a baud uncertainty region and correcting initial timer phase accordingly. A baud timer is advanced or retarded in phase by fixed increments in response to an error signal. Although such a scheme is inadequate for acquiring timing signals from the QPSK header portion of a signal, it is inadequate if a disturbance requires re-acquisition of the timing signal after the modem is in the QAM transmission mode.